Bird Faunas of the Humid Montane Forests of Mesoamerica: Biogeographic Patterns and Priorities for Conservation

Bird Conservation International (1995) 5:251-277 Bird faunas of the humid montane forests of Mesoamerica: biogeographic patterns and priorities for conservation BLANCA E. HERNANDEZ-BANOS, A. TOWNSEND PETERSON, ADOLFO G. XAVARRO-SIGUENZA, and B. PATRICIA ESCALANTE-PLIEGO Summary The distribution of 335 species of birds in 33 islands of humid montane forest in Mesoamerica is summarized, and patterns of distribution, diversity and endemism are analysed. The montane forests of Costa Rica and western Panama far exceed other habitat islands considered for species-richness, richness of species endemic to Mesoamerica, and richness of species ecologically restricted to humid montane forests. Other regions, such as the Sierra Madre del Sur of Guerrero and Oaxaca, the Los Tuxtlas region of southern Veracruz and the mountains of Chiapas and Guatemala, also hold rich and endemic avifaunas. Based on patterns of similarity of avifaunas, the region can be divided into seven regions holding distinctive avifaunas (Costa Rica and western Panama; northern Central America and northern Chiapas; southern Chiapas; eastern Mexico north of the Isthmus of Tehuantepec; Sierra Madre del Sur; interior Oaxaca; and Transvolcanic Belt and Sierra Madre Occidental), which serve as useful guides for the setting of priorities for conservation action. Se resumen las distribuciones de 335 especies de aves en 33 islas de bosque humedo de montana en Mesoamerica, y se analizan patrones de distribution, diversidad y endemismo. Los bosques montanos de Costa Rica y del oeste de Panama tienen la mas alta riqueza de especies, riqueza de especies endemicas a Mesoamerica, y riqueza de especies ecologicamente restringidas a bosque humedo de montana. Otras regiones, tales como la Sierra Madre del Sur de Guerrero y Oaxaca, la region de Los Tuxtlas y las montanas de Chiapas y Guatemala, tambien tienen avifaunas ricas en especies y en endemicas. Basado en patrones de similitud de avifaunas, se puede dividir Mesoamerica en siete regiones que tienen avifaunas distintas (Costa Rica y el oeste de Panama; el norte de Centroamerica y el norte de Chiapas; el sur de Chiapas; el este de Mexico; la Sierra Madre del Sur; el interior de Oaxaca; y el Eje Neovolcanico y la Sierra Madre Occidental), las cuales pueden servir como guias en el establecimiento de prioridades para la conservation. Introduction Tropical forests are rapidly disappearing from the face of the earth, and with them are vanishing multitudes of species of plants and animals that depend on them for food and shelter. Reductions in forest cover in this century are frighteningly large, and have accelerated greatly in recent years owing to Blanca E. Herndndez-Banos et al. 252 expanding human populations, worsening economic situations, and improved technology for forest removal. For this reason, tropical forests have become the focus of intense interest from conservationists working to avoid large-scale loss of biological diversity (Sarukhan and Dirzo 1992). In spite of the interest in preserving tropical forest biodiversity, the geographic distribution of that diversity is still largely unknown (Peterson et al. X993)- That is, for most taxonomic groups, very little is known about distributional limits of species, patterns of species-richness, or patterns of distribution of unique or endemic forms (Soule 1990, Escalante-Pliego et al. 1993, Peterson et al. 1993). Birds are an important group for such studies because their taxonomy is well known, and because inventory methods are well developed, permitting detailed inferences from distributional data (e.g. ICBP 1992). Among the purposes of this paper, therefore, are to summarize the current knowledge of the distribution of birds in one habitat type - humid montane forests - in Mesoamerica, to analyse that information with regard to biogeographic patterns, and to begin to outline strategies for conservation action. Humid montane forests in Mesoamerica For this paper, we define humid montane forest as middle-to-high elevation tropical forest that receives strong input of moisture from rain, fog, and clouds. This definition includes true cloud-forest, which is a dense mixture of temperate and tropical broad-leaved tree species with many epiphytes and tree ferns, as well as humid pine-oak forest. Tree species characteristic of northern Mesoamerican cloud-forests include members of the genera Quercus, Juglans, Podocarpus, and especially Liquidambar (east) and Chiranthodendron (west; Rzedowski 1986). Many genera and species of trees in eastern Mexican cloud-forests are also found in the eastern United States, probably indicating an ancient moist forest connection between the two regions (Martin and Harrell 1959). Humid pine-oak forests grade into true cloud-forests, and share with them structural features such as the abundance of tree ferns and epiphytes (Rzedowski 1986). Faunas of the two habitat types, when in contact, are almost identical (Navarro-Siglienza 1992, Torres-Chavez 1992), and therefore are considered together herein (Escalante-Pliego et al. 1993, Peterson et al. 1993). Humid montane forests are found in broken strings along both coastal slopes of Mesoamerica. This distribution makes intuitive sense: coastal mountains intercept moisture from clouds rolling in off the ocean. These forests are better developed in the east, where climatic conditions are more moist. The degree of development of cloud-forests in a particular region also depends on factors such as soil type and predominant weather patterns, but especially on the presence of high or steeply ascending mountains close to the coast. Hence, the geographic distribution of the habitat is everywhere fragmented and local (Figure 1). The dominant geographic features in Mexico are coastal ranges ("cordilleras") and interior basins. The western Cordillera consists of the Sierra Madre Occidental in the north-west, part of the Transvolcanic Belt from Jalisco south to Michoacan, the Sierra Madre del Sur of Oaxaca and Guerrero, and the Sierra Madre de Chiapas. The eastern Cordillera includes the Sierra Madre Oriental in the north-east; part of the Transvolcanic Belt in Hidalgo, Puebla, and Veracruz; Bird faunas of Mesoamerican humid montane forests 253 \ c A,f ^7 .TAMPS S1NDU \ \ ^^^ HIDAL yy //^MALIN NAYJAt-V—\ /ACVERA '— v /-- r TAN C1—5v'~- . MICHO /\ Jtf/ GUACU TEMAS TAXCO y .NICAR YUCUN//T GU ERR / > ^T [ /CRGUA CUATR' KIAHU / P^f-^ /, CRTIL - ' //CRCEN // / //.CRTAL 1 ^ 1 KONDU ^ GUAPA 1 Figure I. Map of the distribution of patches of humid montane forest in Mesoamerica, showing those included in this study and several that are yet to be studied. the Nudo de Zempoaltepetl of Oaxaca; and the Sierra Norte de Chiapas. Separating these two mountain chains are arid interior basins including the Chihuahuan Desert, Valle de Tehuacan, Valle de Oaxaca, and the interior basins of Chiapas. Farther south, in southern Central America, the topography is often more simple, with one central cordillera and lowlands along both coasts. An east-west transect across Mexico at almost any latitude therefore crosses a remarkably consistent set of vegetation types. First, one encounters wet tropical habitats in the Atlantic coastal lowlands. On the coastal slope of the first range of mountains is a narrow band of cloud-forest, and above that a band of humid pine-oak forest: these two habitats are the focus of the present paper. At the highest elevations, alpine pine-fir forest is found. In the interior basins, dry pine-oak forest, desert, or deciduous tropical scrub prevails. Descending the western slope, the same sequence of habitats is found, from alpine pine-fir forest through humid pine-oak and cloud-forest down to lowland tropical forests. However, because climates of the western lowlands are often less humid than those of the eastern slope, the west-slope tropical forests rarely approach the lush, evergreen quality of the east slope, and indeed are in many cases wholly or partly deciduous. Much of the geography of habitats in the country can be understood on the basis of this simplified scheme. Distributions of vegetation types in Central America are similar, except that in many areas the dry interior basins are lacking, and humid montane forest is therefore more continuous (Long 1995). Blanca E. Herndndez-Banos et al. 254 Mesoamerican humid montane forest avifaunas The birds of humid montane forests in Mesoamerica represent a surprisingly diverse aggregation of species, and rank high among habitats in Mexico in total species-richness (Navarro-Sigiienza 1992, Escalante-Pliego et al. 1993, Peterson et al. 1993, Long 1995). Among the birds present are species commonly associated with lowland rainforests (e.g. Ornate Hawk-eagle Spizaetus ornatus, Ochre-bellied Flycatcher Mionectes oleagineus), as well as species typical of high-elevation, alpine forests (e.g. Sharp-shinned Hawk Accipiter striatus, Golden-crowned Kinglet Regulus satrapa). Also present, and perhaps most interesting, are many species and genera that are restricted to the humid montane forest zone, such as toucanets of the genus Aulacorhynchus, several rare and endangered guans (genera Penelopina, Oreophasis, and Chamaepetes), and jays of the genus Cyanolyca. This variety suggests several ideas about why humid montane forests hold such diverse avifaunas (Escalante-Pliego et al. 1993). One possibility is that they represent a meeting ground between habitats, and their avifaunas largely consist of mixtures of both lowland and highland elements. A second view is that Mesoamerica

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